Occupant detection and classification are frequently used in connection with air bags and other pyrotechnically deployed restraints for purposes of determining if the restraints should be deployed in the event of sufficiently severe crash. Various vehicle seat sensors have been devised for this purpose, such as disclosed, for example, in the U.S. Pat. Nos. 5,987,370 and 6,246,936 to Murphy et al., and the U.S. Pat. Nos. 6,101,436 and 6,490,936 to Fortune et al., all of which are assigned to Delphi Technologies, Inc., and incorporated herein by reference. In general, occupant presence can be detected based on the overall force applied to the seat, while reliably classifying the occupant additionally requires information about the distribution of the applied force.
The information required for occupant classification can be acquired in a cost-effective manner with a capacitive sensor array having a set of conductor plates separated by a compressible dielectric material. An electronic controller detects changes in capacitance between the plates when the thickness of the intermediate dielectric material changes due to the forces applied to the seat by an occupant. See, for example, the U.S. Pat. Nos. 4,836,033 to Seitz; 5,878,620 to Gilbert et al.; 6,448,789 and 6,591,685 to Kraetzl et al.; and 6,499,359 to Washeleski et al. As shown in the U.S. Pat. No. 4,836,033, for example, the distribution of the applied force can be determined by configuring one of the main conductor plates as an array of individual charge plates, and measuring changes in capacitance between each of the individual charge plates and the other plate.
Ideally, the dielectric material of a capacitive sensor should be relatively thin and locally compressible, and should have a dielectric constant that is not significantly influenced by variations in ambient humidity or temperature. The aforementioned U.S. patent application Ser. No. 10/643,044 discloses a particularly advantageous capacitive sensor in which the dielectric is implemented with a fluid-filled elastomeric bladder. When occupant-related seat force is applied to the sensor, fluid in the loaded region of the bladder is displaced to another region where the bladder can expand to accommodate the displaced fluid. However, as the area over which the occupant force is applied increases, the area of the bladder that is able to expand decreases, disturbing the relationship between capacitance change and applied force.